The constant volume sampler (i.e. CVS) was first applied in the late 1950's to make possible the measurement of the mass of exhaust emissions. Before that time, emissions tests had been based on concentration limits. Since the effect on the environment is assessed by the grams of pollutants emitted by vehicles per mile driven, a sampling system was needed that could measure the mass of these emissions while the vehicle was operated through a sequence of accelerations and decelerations that approximated normal driving.
The CVS and its operation are illustrated in FIG. 1 wherein vehicle exhaust enters the CVS at inlet 10, ambient air for dilution enters at 12, and ambient bags and sample bags are indicated at 14 and 16, respectively. The CVS also includes a sample flow pump and a controller 18, a measuring device 20 such as a CFV or SAO to measure CVS flow to calculate volume, and a blower or pump 22 to set CVS flow rate.
All of the vehicle's exhaust is diluted with ambient air in the relatively large mixing "T" of FIG. 1. The combined gases are drawn through the system by the blower or pump 22 at a relatively constant combined flow rate. Thus, a CVS operates at a variable dilution ratio. As the vehicle produces more exhaust, less ambient air is mixed with it in order to keep the total flow constant.
The measuring or metering device 20 in the bulk stream determines the flow rate. In this manner, the total volume of the mixture is easy to determine from the time of the sampling multiplied by the constant flow rate. Today, CVS units actually do not quite operate at a constant rate of flow, but the name CVS is still used.
A small, proportional sample of the diluted gases is collected in the sample bags 16 during the sampling period. The sample bags 16 are analyzed later for the concentrations of the pollutants. A simultaneous sample of the dilution air is also collected in the bag 14 for subsequent analysis.
There are several types of CVS units available today, including fixed flow PDP and CFV types as well as variable flow SAO types.
The requirements of testing low emission vehicles make better sampling systems necessary. Vehicles today may use alternative fuels such as Methanol, CNG and LPG. Burning these fuels produces more water, requiring more dilution, which lowers sample concentrations.
However, the same vehicles are likely to be LEVs and ULEVs, low and ultra low emission vehicles. The emissions from these vehicles are very low, and the concentrations have become very difficult to measure accurately. It is clearly important not to over-dilute these samples, making them even more difficult to measure.
To meet these conflicting requirements, a CVS needs to operate at the correct flow rate for each vehicle, fuel and ambient condition. CVS units based on fixed flow metering orifices however, are only able to operate at a relatively small number of fixed flow rates.
At this point, the CVS method is very near its limit of capability for accurate measurement of low amounts of pollution. The basic limitation arises from the method for diluting the exhaust gases. The only diluent available in sufficient quantity to be practical in a CVS is the ambient air in the test cell. However, this gas already contains considerable water and has background concentrations of the pollutants that can be as large as the concentrations coming from the vehicle. So the CVS must dilute the exhaust gases more than necessary with contaminated diluent. The sample concentrations that result are too low to be conveniently analyzed with conventional gas analyzers and the need to compensate for an almost equal amount of pollutant contributed by the background doubles the uncertainty of the measurement.
Mini-diluters are a new class of devices that avoid these limitations by reversing the order of the diluting and sampling of the exhaust gases, as shown in FIG. 2. The mini-diluter of FIG. 2 includes a pump 24 and sample bags 26.
As previously mentioned, while a CVS system must dilute all of the vehicle's exhaust at a variable dilution ratio, and then take a proportional sample, a mini-diluter reverses this process by first taking a small sample of the exhaust gas and then accurately diluting it at a relatively constant dilution ratio. Since only a small volume of diluent is needed, a dry, contaminant free gas, dry air or nitrogen, can be used. Two advantages are the higher, more easily measured concentrations resulting from less dilution and the absence of background contaminants, eliminating a need for a separate, error prone analysis of the background pollution. The collection and analysis of samples of the diluent air is eliminated, doubling the accuracy of the mass calculation.
A challenge for the mini-diluter is that the rate of collected sample over a test period must be kept proportional to the raw exhaust flow from the vehicle, which is strongly varying, instead of the bulk stream flow through the CVS, which is relatively constant.
The following table illustrates the improvement in accuracy that can be expected with a mini-diluter. It shows the expected sample bag concentrations for a vehicle getting 25 mpg tested at 50% relative humidity at 74.degree. F. that meets the required ULEV emissions levels. The table compares the resulting bag concentrations for a standard CVS with fixed flow at 320 cfm, as referenced in the Code of Federal Regulations, a variable flow CVS, optimized for these conditions, and for a mini-diluter.
______________________________________ Expected Bag Concentrations ULEV Limit CVS VFCVS Mini-Diluter ______________________________________ HC 0.040 g/mi 1.1 ppm 1.9 ppm 3.3 ppm CO 1.70 g/mi 22.7 ppm 39.1 ppm 70.1 ppm NO.sub.x 0.20 g/mi 1.6 ppm 2.8 ppm 5.0 ppm ______________________________________
It can be observed that the mini-diluter technique raises the bag concentrations enough that it is feasible to measure these levels with the same analyzer technology that is in use today.
All these sampling systems, fixed and variable flow CVS's as well as mini-diluter, must perform the following three functions:
Prevent condensation of water in the sample before it can be measured. The water content creates two problems. First, water condensing during the analysis process changes the concentration or the volume of the sampled gases, affecting the accuracy of the result. Second, some contaminants, such as formaldehyde and NO.sub.x, are affected by the removal of water. PA1 Measure the total gas volume over a sampling interval, so the mass of emissions can be calculated. PA1 Collect a proportional sample of diluted exhaust in a sample bag for analysis. At any time, the rate of flow of sampled gases and the total rate of flow of diluted exhaust through the CVS must be in the same proportion.
An improved mini-diluter of the above-noted patent application is illustrated in FIG. 3. The mini-diluter of FIG. 3 includes critical flow orifices (CFO's) 28 to establish a stable dilution ratio. One of the orifices 28 is for the diluent, either nitrogen or dry air, and the other of the orifices 28 is smaller and is for the sample gas. The orifices 28 operate on the principle from fluid dynamics that the flow through an orifice reaches a known maximum flow when the pressure drop across it is large enough that the velocity in the orifice throat reaches the speed of sound. The two orifices 28 are appropriately sized to provide a fixed dilution ratio appropriate for the type of fuel being used.
A modified pressure regulator 30 of the mini-diluter maintains equal pressure at the inlets of the two orifices 28, even as the conditions at the sampling point may change. A reference port is connected to the sample inlet and the action of the regulator 30 keeps the pressure at the inlet of the diluent orifice equal to the pressure at the inlet to the sample orifice.
The wet raw gases are brought to the dilution component via heated lines, and the orifices 28 and pressure regulator 30 are kept in an oven 32 to prevent any condensation of the sample before it is diluted. The oven 32 also keeps the inlet temperatures of both orifices 28 at the same temperature. Together with the action of the pressure regulator 30, this keeps the dilution ratio relatively constant.
The diluted gases can then be pumped by a pump 34 and then routed to either a conventional analyzer bench 36 for a modal analysis or sent to sample bags 38. When used for modal sampling, the mini-diluter replaces the usual sample conditioning unit, providing the advantages of much less extracted sample and no modification of the sample by a cooler.
A mass flow controller 40 having a control valve (not shown) is used to proportion the flow to the bags 38. The absolute accuracy of the mass flow controller 40 is not critical, only that its flow be kept in proportion to the vehicle exhaust flow. It is also important to compensate for the gas transport delays from the sample point to the flow controller 40 so that the desired weighting of the collected gases is correct.
As noted above, one approach to implementing a mini-diluter has been to use mass flow controllers, such as commonly used in the semiconductor industry, to control the rates of flow of raw exhaust sample, diluent gas and proportional diluted sample. However, early attempts have not yet been entirely successfully demonstrated to be equivalent to the CVS method. The stability of the mass flow controllers and the sensitivity of their controlled flow rate to the composition of the flowing gases, as well as their slower response times to sudden changes in desired flow, have presented difficulties.
One of the difficulties of sampling exhaust, which others may not fully appreciate, is the changing water content of the sample. A key realization is that water is lost from the exhaust gas even inside the vehicle exhaust manifolds and piping, before it is even available to be sampled, even by a mini-diluter using heated sampling lines. An engine is initially cold when testing begins and it is during this relatively brief period that most of the pollutants are emitted. It is also during this period that the inaccuracies caused by lost water vapor from condensation in the exhaust manifolds and tailpipes have their greatest influence on the performance of the active elements of the mini-diluter.
The critical flow orifices 28 of FIG. 3 are affected by the changing water content of the exhaust gases, but to a much lesser extent than are the thermo-dynamic elements of a mass flow controller. The mass flow controller 40 of FIG. 3 is utilized only to proportion the sample to the exhaust flow. The absolute accuracy of the flow is not important, since this flow does not need to be measured.